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chem > Energetics I > Flashcards

Energetics I Flashcards

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1
Q

Standard Conditions - temp pressure and conc

A

Temp: 25 degrees celsius / 298K
Pressure: 1 bar / 10^5 Pa
Conc of aq solute: 1mol/dm^3

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2
Q

Defn of enthalpy and symbol

High enthalpy = less/more stable?

A

Enthalpy (H) is a measure of energy content of a sub, hence reflecting its stability
High enthalpy = less stable

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3
Q

Eqn for enthalpy

A

ΔHr = Σ (H prod / final) - Σ (H reactants / initial)

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4
Q

ΔH > 0

  • meaning
  • energy profile diagram
  • energy lvl diagram
  • prod or reactants are more stable?
A

Endothermic - energy absorbed from surroundings & temp of surrounding decrease
H prod > H rcts
Products are less stable than rcts

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5
Q

ΔH < 0

  • meaning
  • energy profile diagram
  • energy lvl diagram
  • prod or reactants are more stable?
A

Exothermic - energy released to surroundings & temp of surrounding increases
H prod < H rcts
Products are more stable than rcts

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6
Q

Defn of activation energy & symbol

A

Activation energy (Ea) is the min. energy the rct molecules must possess in order to collide successfully to form prod. It affects the speed of the rctn

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7
Q

Standard enthalpy change of rctn & symbol

A

ΔHrΘ

The energy change when molar quantities of rcts specified in the chemical eqn reacts to form prod at 1 bar & 298K

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8
Q

All chem eqn must have …

A

State symbols

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9
Q

Measurement of Standard enthalpy change of rctn depends on (2)

A
  1. Amt of rcts and prod
    - if they are X2, then ΔH X2
    - order is reversed then sign is opp
  2. Phy state of the sub
    - Prod (l) is more stable than prod (g) so lower
    - Diamond is also more stable than graphite so lower
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10
Q

Defn of Hess’ Law

A

The enthalpy change accompanying a chem rctn is the same regardless of the route by which the chem change occurs, provided the initial and final conditions are the same.

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11
Q

5 steps of Hess’ Law

A
  1. Write balanced eqn
  2. start energy cycle with most complicated eqn
  3. complete cycle by adding in eqns….
  4. Write the corresponding ΔH values next to the arrows
  5. Sum of CW arrows = sum of ACW arrows
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12
Q

Defn of standard enthalpy change of formation
symbol
eqn
Common elements*
For an element at std state …
Stability of a sub relative to the constituent elements

A

ΔHfΘ is the enthalpy change when 1 mole of a species is formed from its constituent elements in their std states at 298K and 1 bar
ΔHrΘ = Σ HfΘ (prod) - Σ HfΘ (reactants) FLAPPER
Common elements: P4 (s), Br2 (l), S8 (s)
For an element at std state, ΔHfΘ = 0
If ΔHfΘ < 0, then sub is more stable than the elements & vice versa

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13
Q

Defn of standard enthalpy change of combustion
symbol
eqn
For org compounds with only C / H / O, prod are always…

A

ΔHcΘ is the heat evolved when 1 mole of a sub is completely burned in excess oxygen under std conditions of 298K and 1 bar
ΔHrΘ = Σ HcΘ (rcts) - Σ HcΘ (prod) CRAP
Prod are always CO2 (g) + H2O (l)
Always exo

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14
Q

Defn of bond energies
symbol
eqn

A

Bond energies is the ave energy absorbed when breaking 1 mole of covalent bond in the gaseous state
ΔHrΘ = Σ BE (rcts / bond broken) - Σ BE (prod / bonds formed) BURP

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15
Q

Requirement to use bond energies in calc

A

Rctn must take place in the gaseous state

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16
Q

Why might there be discrepancies in BE in DB

A

BE in DB are averaged across a large sample of molecules containing the covalent bond

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17
Q

Qn eg:

Using relevant BE values in DB calc… state and explain if the and in (a) is an estimate/exact value

A

Exact value as the BE used in calc are exact values obtained from the DB as HCL is a heteronuclear molecules, while H2 and CO2 are homonuclear molecules and all in gaseous state

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18
Q

Defn of bond dissociation energy of a X-Y bond

A

Energy required to break 1 mole of that particular X-Y bond in a particular compound in gaseous state

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19
Q

Defn of standard enthalpy change of hydration
symbol
Mag depends on
eqn eg

A

ΔHhydΘ is the energy released when 1 mole of a gaseous ion is dissolved in a large amt of water at 298K and 1 bar
|ΔHhydΘ| ∝ |charge of ion/size of ion| (charge density)
Na+ (g) -> Na+ (aq)

20
Q

Why is std enthalpy change of hydration is exo

A

Energy is released during the formation of ion-dipole interactions
Larger charge density - more exo ΔHhydΘ - stronger ion-dipole interactions

21
Q

Defn of std enthalpy change of soln
symbol
soluble/insoluble

A

ΔHsolΘ is the enthalpy change when 1 mole of solute is completely dissolved in a solvent to form an infinitely dilute soln under std conditions of 298K and 1 bar
(usually a solid ionic compound)
ΔHsolΘ&raquo_space; insoluble in water
ΔHsolΘ < soluble in water

22
Q

Defn of lattice energy
Symbol
Mag depends on

A

ΔHlattΘ is the heat evolved when 1 mole of a solid ionic compound is formed from its constituent gaseous ions under std conditions of 298K and 1 bar
|ΔHlattΘ| ∝ | q+ x q- / r+ + r- |

23
Q

Eqn for ΔHsolΘ

A

ΔHsolΘ = Σ ΔHhydΘ - Σ ΔHlattΘ

fav soln is hyd latte

24
Q

Is Lattice energy exo/endo and why

A

Exo as

Energy is released when strong electrostatic forces of attraction are formed b/w cations and anions

25
Q

How to describe ΔH

A

ΔH is less -ve/+ve OR less exo/endo

NOT larger/smaller

26
Q

Defn of std enthalpy change of atomisation of an element
Symbol
For noble gases…

A

ΔHatΘ is the std enthalpy change of atomisation is the energy absorbed when 1 mole of gaseous atoms is formed from its element under std conditions of 298K and 1 bar
For noble gases, ΔHatΘ = 0 since they already exist as gaseous atoms

27
Q

Is std enthalpy change of atomisation exo or endo and why

A

Endo as energy is absorbed to break covalent or metallic bonds

28
Q

R/s of std enthalpy change of atomisation and bond energy for diatomic molecules

A

2 ΔHatΘ = BE

29
Q

Defn of first ionisation energy

A

Energy absorbed to remove 1 mole of e- from 1 mole of gaseous atom of the element to form 1 mole of singly charged gaseous cation

30
Q

IE is exo/endo? and why

A

Endo as energy is required to overcome the electrostatic attraction b/w the nucleus and the e- being removed

31
Q

Defn of first e- affinity

A

Energy change when 1 mole of e- is added to 1 mole of gaseous atoms to form 1 mole of singly negatively charged gaseous ions (usually exo)

32
Q

Defn of 2nd ionisation energy

A

The 2nd ionisation energy of an element is the energy absorbed to remove 1 mole of electrons from 1 mole of singly positively charged gaseous ions to form 1 mole of doubly positively charged gaseous ions

33
Q

Defn of 2nd electron affinity

A

Is the energy absorbed when 1 mole of e- is added to 1 mole of singly -vely charged gaseous ions to form 1 mole of doubly -vely charged gaseous ions

34
Q

Why is 1st e- affinity exo/endo and 2nd e- affinity exo/endo

A 
1st: likely to be exo because energy is released from the electrostatic attraction b/w the incoming e- and nucleus 
smaller atoms (eg grp 17) have higher electrostatic attraction

2nd: Endo as energy is required to overcome the electrostatic repulsion b/w the anion and the 2nd 2- gained

35
Q

Born haber cycle general steps

A 
FAIEL: 
Formation
atomisation
Ionisation
E- affinity
LE
36
Q

Zero reference line in born-haber cycle

A

For the elements in their std states

37
Q

How does extrapolation work

and if curve has no curve, u are assuming

A

Assuming that:

  • negligible heat loss/gain to /from surroundings
  • calorimeter absorbs a negligible amt of heat
  • rctn is rapid so max/min temp can be reached before the mixture returns to initial temp
38
Q

Defn of heat capacity and unit

A

Qty of heat required to raise its temp by 1K or 1 degrees celsius
K J-1

39
Q

Defn of specific heat capacity and unit

A

Qty of heat required to raise its temp of a unit mass (1g) by 1K or 1 degrees celsius
K g -1 J-1

40
Q

2 eqns for calc heat transactions

A 
q = mcΔT = CΔT
ΔHr = -q/n'
q = heat transacted
n' = amt of limiting rct
41
Q

Percentage efficiency

A

exp heat transaction/theoretical heat transaction X 100%

42
Q

When solving qn with certain efficiency use this:

A

E . q1 = -q2

where E is the efficiency, q1 = ΔH X n’ and q2 = mcΔT

43
Q

When solving qn where calorimeter also absorbs heat

A

eg

q = (mcΔT) H2O + (mcΔT) calorimeter

44
Q

Defn of std enthalpy change of neutralisation
symbol
Value for strong acids & bases and why it’s the same for all
Why is the value ^ diff for weak acid/base

A

ΔHneutΘ is the energy changed when 1 mole of water is formed from the neutralisation of an acid and a base at 298K and 1 bar
Values = -57.3 KJ mol-1
as they are all completely ionised and net rctn is b/w 1 mol of H+ and 1 mol of OH-

Value of less exo than -57.3 as
partially ionised in soon
some amt of energy is required to cause ionisation of the undissociated weak acid molecules in order for neutralisation to occur
Thus, net energy released during neutralisation will be less

45
Q

Weak acids/base usually contain what element

A

Acid - C

Base - N

46
Q

Why is the experimental LE and theoretical LE diff?

A

Theoretical LW is calc using an eqn based on the purely ionic model of a lattice

Ionic compounds with partial covalent character have exp LE > theoretical

The discrepancy b/w …. shows that the bonding in the mg halides and Ag halides is not as close to ‘purely ionic’ as it is with the sodium halides. These ionic compounds have some covalent character. This covalent character arises due to substantial polarisation of the anion by the cation. The exp LE is more negative. This indicates that the bonding in these compounds is, in fact, stronger than that predicted by the purely ionic model

chem (13 decks)

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